Need help with new headphones and staticy/fuzzy noise coming from a USB DAC

I'm technically a new user here, but I've been reading through head-fi threads/reviews for headphones for several years now, and have made a couple of purchases based on people's recommendations that have been really useful.

Background: My current headphones are a pair of Creative Aurvana Lives that I purchased about a year and a half ago. I've always had (and still do, to a certain extent) budgetary constraints for headphones, so these are the nicest pair of headphones I've ever had. I use them to listen to music on my laptop (which has crummy on-board audio), my desktop (which also has crummy on-board Audio), and my iPhone 4S.

When I got the headphones, I started to notice that music on my laptop sounded kinda fuzzy/staticy/hissy, especially when I listened to quieter songs (it's very noticeable on Windows system sounds, for instance). At the time, I attributed the noise to either a crummy sound card or some electrical noise from the motherboard (I did a test for latency and it came up clean) so I bought a Creative X-Fi Go USB DAC (the $20 - $30 one). Though the audio quality is certainly better, the hiss is very much still there. The same situation has presented itself with my desktop (so I reallyyyy don't think it's the case that both my computers have noisy motherboards), and the USB DAC has the same hiss when I use with this system, as well. The sound out of my iPod, though, is clear and crisp.

I understand that USB DACs are far from a perfect solution - they're cramped and, in my case, cheap, so it's not reasonable to expect much (though I honestly don't understand how the presumably small and cheap sound output of my iPhone sounds better...) Where do I go from here? Can I reasonably expect a better USB DAC to solve the problem? And, if so, what should I get? Do I need something that amplifies, as well? I'd also appreciate some insights into what causes the problem, as well (I've read a lot about impedance mismatching but the CALs don't seem to be low enough impedance that the argument applies...)